This invention relates to a composition for suppressing electromagnetic radiation and, particularly, for reducing the reflection of microwave energy.
The use of materials for absorbing electromagnetic radiation is wide spread in the coating of (1) military devices which are required to avoid or minimize detection by radar, (2) appliances that employ microwave radiation, and (3) reflectors of ships, airplanes, building and bridges to reduce reflection that often causes navigational errors.
Many materials including natural ones and synthetic ones are known for their ability to surpress electromagnetic radiation in the microwave frequency range. This ability to suppress electromagnetic radiation enables the absorbing material to dissipate electromagnetic energy within the material, thereby reducing the reflection of microwaves.
Of the various absorbing materials, the artificial dielectrics are the most commonly employed. Artificial dielectrics are generally formed by dispersing a magnetic powder or other natural absorber in a dielectric material, such as plastics including thermoplastics and thermosets, ceramics, waxes and the like. The artificial dielectrics which have been formed by loading the aforementioned dielectric binders with magnetic metals, semi-conductors, ferromagnetic oxides or ferrites have very desirable magnetic and dielectric properties.
The use of solid ferrites, i.e., ferromagnetic ferrites formed of ferric oxide and other bivalent metal oxides, as sheet materials for reflecting surfaces and objects to suppress or substantially reduce the reflection of electromagnetic energy offers many advantages. It has been found that mixed ferrites often provide good absorptive materials over a wide range of microwave frequencies. In addition, ferrites in the form of solid coatings display the higher permeabilities which are required for broad band operation. Such solid ferrite coatings are capable of higher permeabilities than those exhibited by the ferrite powders since the magnetic properties of ferrite decline appreciably by grinding it into powder form. Thus, it is found that ferrites that are both non-conductive and ferromagnetic provide within a single composition the potentially optimum dielectric and magnetic properties.
Unfortunately, it is found that, in the conventional absorptive coatings that contain ferrites, substantial quantities of the heavy ferrites is required in order to achieve the desired absorptive capability. The resulting dense coatings of such conventional absorbers are generally undesirable because they are heavy and difficult to fabricate.
In view of the aforementioned deficiencies of the prior art materials for absorbing electromagnetic radiation, it is highly desirable to provide a lightweight absorptive material that can be readily fabricated into any shape or applied as a coating to any of a variety of substrates which coatings contain relatively low concentrations of the heavy magnetic particles needed for absorption.